considered
in relation to the areas over which theyare
applied. As previously discussed, a forceacting
over a unit area is a pressure, and pressurecan
alternately be stated in pounds per square inchor
in terms of head, which is the vertical heightof
the column of fluid whose weight wouldproduce
that pressure.

In most of the applications of fluid power in

the
Navy, applied forces greatly outweigh all otherforces,
and the fluid is entirely confined. Underthese
circumstances it is customary to think of theforces
involved in terms of pressures. Since theterm
head is encountered frequently in the studyof
fluid power, it is necessary to understand whatit
means and how it is related to pressure andforce.

All five of the factors that control the actions

of
fluids can, of course, be expressed either asforce,
or in terms of equivalent pressures or head.In
each situation, the different factors are referredto
in the same terms, since they can be added andsubtracted
to study their relationship to eachother.

At this point you need to review some terms

in
general use. Gravity head, when it is importantenough
to be considered, is sometimes referredto
as head. The effect of atmospheric pressure isreferred
to as atmospheric pressure. (Atmosphericpressure
is frequently and improperly referred toas
suction.) Inertia effect, because it is alwaysdirectly
related to velocity, is usually calledvelocity
head; and friction, because it representsa
loss of pressure or head, is usually referred toas
friction head.

STATIC AND DYNAMIC FACTORS

Gravity, applied forces, and atmospheric

pressure
are static factors that apply equally tofluids at rest or in motion, while inertia andfriction
are dynamic factors that apply only tofluids
in motion. The mathematical sum ofgravity,
applied force, and atmospheric pressureis
the static pressure obtained at any one pointin
a fluid at any given time. Static pressure existsin
addition to any dynamic factors that may alsobe
present at the same time.

Remember, Pascal’s law states that a pressure

set
up in a fluid acts equally in all directions andat
right angles to the containing surfaces. Thiscovers
the situation only for fluids at rest orpractically
at rest. It is true only for the factorsmaking
up static head. Obviously, when velocitybecomes
a factor it must have a direction, andas
previously explained, the force related to thevelocity
must also have a direction, so thatPascal’s
law alone does not apply to the dynamicfactors
of fluid power.

related
to the static factors. Velocity head andfriction
head are obtained at the expense of statichead.
However, a portion of the velocity head canalways
be reconverted to static head. Force, whichcan
be produced by pressure or head when dealingwith
fluids, is necessary to start a body movingif
it is at rest, and is present in some form whenthe
motion of the body is arrested; therefore,whenever
a fluid is given velocity, some part ofits
original static head is used to impart thisvelocity,
which then exists as velocity head.

BERNOULLI’S PRINCIPLE

Consider the system illustrated in figure 2-18.

Chamber
A is under pressure and is connected bya
tube to chamber B, which is also under pressure.The
pressure in chamber A is static pressure of100
psi. The pressure at some point (X) along theconnecting
tube consists of a velocity pressure of

10 psi exerted in a direction parallel to the line of
flow, plus the unused static pressure of 90 psi,which
still obeys Pascal’s law and operates equallyin
all directions. As the fluid enters chamber Bit
is slowed down, and its velocity is changed backto
pressure. The force required to absorb itsinertia
equals the force required to start the fluidmoving
originally, so that the static pressure inchamber B is equal to that in chamber
A.This situation (fig. 2-18)
disregards friction;therefore, it
would not be encountered in actualpractice.
Force or head is also required toovercome
friction but, unlike inertia effect, thisforce
cannot be recovered again, although theenergy
represented still exists somewhere as heat.Therefore,
in an actual system the pressure inchamber
B would be less than in chamber A bythe
amount of pressure used in overcomingfriction
along the way.

At all points in a system the static pressure is always
the original static pressure, less any velocityhead
at the point in question and less the frictionhead
consumed in reaching that point. Since boththe
velocity head and the friction head representenergy
that came from the original static head,and
since energy cannot be destroyed, the sum ofthe
static head, the velocity head, and the frictionhead
at any point in the system must add up tothe
original static head. This is known asBernoulli's
principle, which states: For thehorizontal
flow of fluid through a tube, the sumof
the pressure and the kinetic energy per unitvolume
of the fluid is constant. This principlegoverns
the relations of the static and dynamicfactors
concerning fluids, while Pascal’s law statesthe
manner in which the static factors behavewhen
taken by themselves.

MINIMIZING FRICTION

Fluid power equipment is designed to reduce friction
to the lowest possible level. Volume andvelocity
of flow are made the subject of carefulstudy.
The proper fluid for the system is chosen.Clean,
smooth pipe of the best dimensions for theparticular
conditions is used, and it is installedalong
as direct a route as possible. Sharp bendsand
sudden changes in cross-sectional areas areavoided.
Valves, gauges, and other componentsare
designed to interrupt flow as little as possible.Careful
thought is given to the size and shape ofthe
openings. The systems are designed so theycan be kept clean inside and variations fromnormal
operation can easily be detected andremedied.

OPERATION OF HYDRAULIC COMPONENTS

To transmit and control power through pressurized
fluids, an arrangement of inter-connectedcomponents
is required. Such anarrangement is
commonly referred to as a system.The
number and arrangement of the componentsvary
from system to system, depending on theparticular
application. In many applications, onemain
system supplies power to several subsystems,which
are sometimes referred to as circuits. Thecomplete
system may be a small compact unit;more
often, however, the components are locatedat
widely separated points for convenient controland
operation of the system.

The basic components of a fluid power system are
essentially the same, regardless of whether thesystem
uses a hydraulic or a pneumatic medium.There
are five basic components used in a system.

Several applications of fluid power require only
a simple system; that is, a system which usesonly
a few components in addition to the fivebasic
components. A few of these applications arepresented
in the following paragraphs. We willexplain
the operation of these systems briefly atthis
time so you will know the purpose of eachcomponent
and can better understand howhydraulics
is used in the operation of thesesystems.
More complex fluid power systems aredescribed
in chapter 12.

HYDRAULIC JACK

The hydraulic jack is perhaps one of the simplest
forms of a fluid power system. Bymoving
the handle of a small device, an individualcan
lift a load weighing several tons. A smallinitial
force exerted on the handle is transmittedby
a fluid to a much larger area. To understandthis
better, study figure 2-19. The small inputpiston
has an area of 5 square inches and isdirectly
connected to a large cylinder with anoutput
piston having an area of 250 square inches.The
top of this piston forms a lift platform.If
a force of 25 pounds is applied to the inputpiston,
it produces a pressure of 5 psi in the fluid,that
is, of course, if a sufficient amount ofresistant
force is acting against the top of theoutput
piston. Disregarding friction loss, thispressure
acting on the 250 square inch area of theoutput
piston will support a resistance force of1,250
pounds. In other words, this pressure couldovercome
a force of slightly under 1,250 pounds.An
input force of 25 pounds has been transformedinto
a working force of more than half a ton;however,
for this to be true, the distance traveledby
the input piston must be 50 times greater thanthe
distance traveled by the output piston. Thus,for
every inch that the input piston moves, theoutput
piston will move only one-fiftieth of ani
n c h.

This would be ideal if the output piston needed to
move only a short distance. However, in mostinstances,
the output piston would have to becapable
of moving a greater distance to serve apractical
application. The device shown in figure2-19
is not capable of moving the output pistonfarther
than that shown; therefore, some othermeans
must be used to raise the output piston toa
greater height.

Figure 2-19.—Hydraulic jack.

The output piston can be raised higher and maintained
at this height if additional componentsare
installed as shown in figure 2-20. In thisillustration
the jack is designed so that it can beraised,
lowered, or held at a constant height.These
results are attained by introducing a numberof
valves and also a reserve supply of fluid to beused
in the system.

Notice that this system contains the five basic components—the
reservoir; cylinder 1, whichserves
as a pump; valve 3, which serves as adirectional
control valve; cylinder 2, which servesas
the actuating device; and lines to transmit thefluid
to and from the different components. Inaddition,
this system contains two valves, 1 and2,
whose functions are explained in the followingdiscussion.

As the input piston is raised (fig. 2-20, view A),
valve 1 is closed by the back pressure fromthe
weight of the output piston. At the same time,valve
2 is opened by the head of the fluid in thereservoir.
This forces fluid into cylinder 1. Whenthe
input piston is lowered (fig. 2-20, view B), apressure
is developed in cylinder 1. When thispressure
exceeds the head in the reservoir, it closesvalve
2. When it exceeds the back pressure fromthe
output piston, it opens valve 1, forcing fluidinto
the pipeline. The pressure from cylinder 1 is

Figure 2-20.—Hydraulic jack; (A) up stroke; (B) downstroke.

thus transmitted into cylinder 2, where it acts to raise
the output piston with its attached liftplatform.
When the input piston is again raised,the
pressure in cylinder 1 drops below that incylinder
2, causing valve 1 to close. This preventsthe
return of fluid and holds the output pistonwith
its attached lift platform at its new level.During
this stroke, valve 2 opens again allowinga
new supply of fluid into cylinder 1 for the nextpower
(downward) stroke of the input piston.Thus,
by repeated strokes of the input piston, thelift
platform can be progressively raised. To lowerthe
lift platform, valve 3 is opened, and the fluidfrom
cylinder 2 is returned to the reservoir.